Scaffold CMOS 18 reformatter: harness, linter, parser, formatter, CLI

Initial phase-1 baseline of the karpathy/autoresearch-style loop.
The formatter module is the inner-loop artifact; parser and linter
are infra. The linter carries a LINTER_VERSION hash (v0.2.0) that
will force a re-baseline on any rule change.

Components:
- harness/diff.py: case-sensitive field-level substring diff
- harness/score.py: three-axis scoring (field, linter, canary exact)
- src/cmos/linter.py: 9 CMOS 18 structural rules, each Purdue/CMOS cited
- src/cmos/parser.py: locate ## Bibliography section, split entries
- src/cmos/formatter.py: prompt + OpenAI call with caller injection
- src/cmos/cli.py: cmos format path/to/draft.md
- scripts/run_loop.py: loop runner with --fake mode for no-API runs
- exemplars/: 3 canary seed exemplars (book, journal w/DOI, web),
  sourced from chicagomanualofstyle.org quick guide

Tests: 48 passing. Fake-mode baseline scalar = 0.000 on the 3 seed
exemplars (identity caller fails the linter on every rule). This is
the floor the real GPT-5 formatter needs to improve from.

harness/score.py

@@ -0,0 +1,122 @@
+"""Frozen scoring harness.
+
+Runs a formatter callable against a list of exemplars and aggregates:
+- field-level match rate (via harness.diff.field_diff)
+- linter pass rate (if a linter function is provided)
+- canary exact-match rate (on exemplars marked canary=True)
+
+The loop scalar is `field_match_rate * linter_pass_rate` (linter defaults to
+1.0 when no linter is wired in yet). Canary exact-match is tracked separately
+and treated as a regression gate at the loop level, not folded into the
+scalar.
+
+This module is FROZEN with respect to the autoresearch loop — edits here are
+infra commits that invalidate prior scores.
+"""
+
+from __future__ import annotations
+
+import tomllib
+from dataclasses import dataclass, field
+from pathlib import Path
+from typing import Callable
+
+from harness.diff import field_diff
+
+
+@dataclass
+class Exemplar:
+    name: str
+    source: str
+    type: str
+    tags: list[str]
+    canary: bool
+    messy_input: str
+    canonical: dict
+    expected_bibliography: str
+
+
+@dataclass
+class ExemplarResult:
+    name: str
+    candidate: str
+    field_match_rate: float
+    linter_passed: bool
+    canary: bool
+    exact_match: bool
+
+
+@dataclass
+class ScoreResult:
+    per_exemplar: list[ExemplarResult] = field(default_factory=list)
+    field_match_rate: float = 0.0
+    linter_pass_rate: float = 1.0
+    canary_exact_match_rate: float = 1.0
+    scalar: float = 0.0
+
+
+LinterFn = Callable[[str, Exemplar], bool]
+FormatterFn = Callable[[str], str]
+
+
+def _mean(values: list[float]) -> float:
+    return sum(values) / len(values) if values else 1.0
+
+
+def score(
+    exemplars: list[Exemplar],
+    formatter: FormatterFn,
+    linter: LinterFn | None = None,
+) -> ScoreResult:
+    """Run `formatter` against each exemplar and aggregate a score."""
+    results: list[ExemplarResult] = []
+    for ex in exemplars:
+        candidate = formatter(ex.messy_input)
+        diff = field_diff(candidate, ex.canonical)
+        linter_ok = True if linter is None else bool(linter(candidate, ex))
+        results.append(
+            ExemplarResult(
+                name=ex.name,
+                candidate=candidate,
+                field_match_rate=diff.rate,
+                linter_passed=linter_ok,
+                canary=ex.canary,
+                exact_match=(candidate == ex.expected_bibliography),
+            )
+        )
+
+    field_rate = _mean([r.field_match_rate for r in results])
+    linter_rate = _mean([1.0 if r.linter_passed else 0.0 for r in results])
+    canary_results = [r for r in results if r.canary]
+    canary_rate = (
+        _mean([1.0 if r.exact_match else 0.0 for r in canary_results]) if canary_results else 1.0
+    )
+
+    return ScoreResult(
+        per_exemplar=results,
+        field_match_rate=field_rate,
+        linter_pass_rate=linter_rate,
+        canary_exact_match_rate=canary_rate,
+        scalar=field_rate * linter_rate,
+    )
+
+
+def load_exemplars(directory: Path) -> list[Exemplar]:
+    """Load every `.toml` file in `directory` as an Exemplar."""
+    exemplars: list[Exemplar] = []
+    for path in sorted(directory.glob("*.toml")):
+        with path.open("rb") as fh:
+            data = tomllib.load(fh)
+        exemplars.append(
+            Exemplar(
+                name=path.stem,
+                source=data["source"],
+                type=data["type"],
+                tags=list(data.get("tags", [])),
+                canary=bool(data.get("canary", False)),
+                messy_input=data["messy_input"],
+                canonical=dict(data["canonical"]),
+                expected_bibliography=data["expected_bibliography"],
+            )
+        )
+    return exemplars